Hermetically sealed space suits typically develop a significant heat load. The heat load must be removed for the comfort of the space suit occupant. The heat load is comprised of several constituents; metabolic heat energy, heat energy absorbed into the suit from the environment, and other electrical and mechanical heat energy. By far, the major portion of the heat load is generated by the occupant's metabolism.
Generally, a liquid cooling garment (LCG), which is designed to remove metabolic heat energy, is worn by the occupant. The LCG generally includes a plurality of tubes strategically placed about the body. The tubes carry a cooling medium, such as water. The cooling medium absorbs the metabolic heat energy and flows to a device which transfers the heat energy therefrom.
Current space suits remove the heat energy generated therein by sublimation. Sublimation occurs when a solid vaporizes without the intermediate formation of a liquid. Water from a separate storage tank is sublimated by exposing it to the vacuum of space. The water is exposed to the vacuum by means of a porous plate which forms part of a heat exchanger. The porous plate traps and freezes the water. The cooling medium, meanwhile, circulates through separate passages within the heat exchanger. The heat energy of the cooling medium sublimates the thin layer of ice producing a small cloud of water vapor. The heating load absorbed by the cooling medium is effectively rejected from the suit thereby. The resulting water vapor is vented to the environment.
While the sublimation process provides the amount of heat removal required, it has several drawbacks: the cost of hauling water into space is prohibitive; the water vapor may condense on mirrors or other sensitive instruments and degrade their performance; and, the water vapor may absorb infrared radiation.